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<main>
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<h1 id="adr-008-tokio-multi-threaded-runtime"><a class="header" href="#adr-008-tokio-multi-threaded-runtime">ADR-008: Tokio Multi-Threaded Runtime</a></h1>
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<p><strong>Status</strong>: Accepted | Implemented
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<strong>Date</strong>: 2024-11-01
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<strong>Deciders</strong>: Runtime Architecture Team
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<strong>Technical Story</strong>: Selecting async runtime for I/O-heavy workload (API, DB, LLM calls)</p>
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<hr />
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<h2 id="decision"><a class="header" href="#decision">Decision</a></h2>
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<p>Usar <strong>Tokio multi-threaded runtime</strong> con configuración default (no single-threaded, no custom thread pool).</p>
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<hr />
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<h2 id="rationale"><a class="header" href="#rationale">Rationale</a></h2>
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<ol>
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<li><strong>I/O-Heavy Workload</strong>: VAPORA hace many concurrent calls (SurrealDB, NATS, LLM APIs, WebSockets)</li>
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<li><strong>Multi-Core Scalability</strong>: Multi-threaded distributes work across cores eficientemente</li>
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<li><strong>Production-Ready</strong>: Tokio es de-facto estándar en Rust async ecosystem</li>
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<li><strong>Minimal Config Overhead</strong>: Default settings tuned para la mayoría de casos</li>
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</ol>
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<hr />
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<h2 id="alternatives-considered"><a class="header" href="#alternatives-considered">Alternatives Considered</a></h2>
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<h3 id="-single-threaded-tokio-tokiomain-single_threaded"><a class="header" href="#-single-threaded-tokio-tokiomain-single_threaded">❌ Single-Threaded Tokio (<code>tokio::main</code> single_threaded)</a></h3>
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<ul>
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<li><strong>Pros</strong>: Simpler to debug, predictable ordering</li>
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<li><strong>Cons</strong>: Single core only, no scaling, inadequate for concurrent workload</li>
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</ul>
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<h3 id="-custom-threadpool"><a class="header" href="#-custom-threadpool">❌ Custom ThreadPool</a></h3>
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<ul>
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<li><strong>Pros</strong>: Full control</li>
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<li><strong>Cons</strong>: Manual scheduling, error-prone, maintenance burden</li>
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</ul>
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<h3 id="-tokio-multi-threaded-chosen"><a class="header" href="#-tokio-multi-threaded-chosen">✅ Tokio Multi-Threaded (CHOSEN)</a></h3>
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<ul>
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<li>Production-ready, well-tuned, scales across cores</li>
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</ul>
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<hr />
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<h2 id="trade-offs"><a class="header" href="#trade-offs">Trade-offs</a></h2>
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<p><strong>Pros</strong>:</p>
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<ul>
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<li>✅ Scales across all CPU cores</li>
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<li>✅ Efficient I/O multiplexing (epoll on Linux, kqueue on macOS)</li>
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<li>✅ Proven in production systems</li>
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<li>✅ Built-in task spawning with <code>tokio::spawn</code></li>
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<li>✅ Graceful shutdown handling</li>
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</ul>
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<p><strong>Cons</strong>:</p>
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<ul>
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<li>⚠️ More complex debugging (multiple threads)</li>
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<li>⚠️ Potential data race if <code>Send/Sync</code> bounds not respected</li>
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<li>⚠️ Memory overhead (per-thread stacks)</li>
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</ul>
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<hr />
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<h2 id="implementation"><a class="header" href="#implementation">Implementation</a></h2>
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<p><strong>Runtime Configuration</strong>:</p>
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<pre><pre class="playground"><code class="language-rust">// crates/vapora-backend/src/main.rs:26
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#[tokio::main]
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async fn main() -> Result<()> {
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// Default: worker threads = num_cpus(), stack size = 2MB
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// Equivalent to:
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// let rt = tokio::runtime::Builder::new_multi_thread()
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// .worker_threads(num_cpus::get())
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// .enable_all()
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// .build()?;
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}</code></pre></pre>
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<p><strong>Async Task Spawning</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// Spawn independent task (runs concurrently on available worker)
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tokio::spawn(async {
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let result = expensive_operation().await;
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handle_result(result).await;
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});
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Blocking Code in Async Context</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// Block sync code without blocking entire executor
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let result = tokio::task::block_in_place(|| {
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// CPU-bound work or blocking I/O (file system, etc)
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expensive_computation()
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});
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Graceful Shutdown</strong>:</p>
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<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>// Listen for Ctrl+C
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let shutdown = tokio::signal::ctrl_c();
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tokio::select! {
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_ = shutdown => {
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info!("Shutting down gracefully...");
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// Cancel in-flight tasks, drain channels, close connections
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}
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_ = run_server() => {}
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}
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<span class="boring">}</span></code></pre></pre>
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<p><strong>Key Files</strong>:</p>
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<ul>
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<li><code>/crates/vapora-backend/src/main.rs:26</code> (Tokio main)</li>
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<li><code>/crates/vapora-agents/src/bin/server.rs</code> (Agent server with Tokio)</li>
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<li><code>/crates/vapora-llm-router/src/router.rs</code> (Concurrent LLM calls via tokio::spawn)</li>
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</ul>
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<hr />
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<h2 id="verification"><a class="header" href="#verification">Verification</a></h2>
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<pre><code class="language-bash"># Check runtime worker threads at startup
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RUST_LOG=tokio=debug cargo run -p vapora-backend 2>&1 | grep "worker"
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# Monitor CPU usage across cores
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top -H -p $(pgrep -f vapora-backend)
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# Test concurrent task spawning
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cargo test -p vapora-backend test_concurrent_requests
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# Profile thread behavior
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cargo flamegraph --bin vapora-backend -- --profile cpu
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# Stress test with load generator
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wrk -t 4 -c 100 -d 30s http://localhost:8001/health
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# Check task wakeups and efficiency
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cargo run -p vapora-backend --release
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# In another terminal:
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perf record -p $(pgrep -f vapora-backend) sleep 5
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perf report | grep -i "wakeup\|context"
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</code></pre>
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<p><strong>Expected Output</strong>:</p>
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<ul>
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<li>Worker threads = number of CPU cores</li>
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<li>Concurrent requests handled efficiently</li>
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<li>CPU usage distributed across cores</li>
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<li>Low context switching overhead</li>
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<li>Latency p99 < 100ms for simple endpoints</li>
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</ul>
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<hr />
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<h2 id="consequences"><a class="header" href="#consequences">Consequences</a></h2>
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<h3 id="concurrency-model"><a class="header" href="#concurrency-model">Concurrency Model</a></h3>
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<ul>
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<li>Use <code>Arc<></code> for shared state (cheap clones)</li>
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<li>Use <code>tokio::sync::RwLock</code>, <code>Mutex</code>, <code>broadcast</code> for synchronization</li>
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<li>Avoid blocking operations in async code (use <code>block_in_place</code>)</li>
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</ul>
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<h3 id="error-handling"><a class="header" href="#error-handling">Error Handling</a></h3>
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<ul>
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<li>Panics in spawned tasks don't kill runtime (captured via <code>JoinHandle</code>)</li>
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<li>Use <code>.await?</code> for proper error propagation</li>
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<li>Set panic hook for graceful degradation</li>
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</ul>
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<h3 id="monitoring"><a class="header" href="#monitoring">Monitoring</a></h3>
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<ul>
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<li>Track task queue depth (available via <code>tokio-console</code>)</li>
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<li>Monitor executor CPU usage</li>
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<li>Alert if thread starvation detected</li>
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</ul>
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<h3 id="performance-tuning"><a class="header" href="#performance-tuning">Performance Tuning</a></h3>
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<ul>
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<li>Default settings adequate for most workloads</li>
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<li>Only customize if profiling shows bottleneck</li>
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<li>Typical: num_workers = num_cpus, stack size = 2MB</li>
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</ul>
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<hr />
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<h2 id="references"><a class="header" href="#references">References</a></h2>
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<ul>
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<li><a href="https://tokio.rs/tokio/tutorial">Tokio Documentation</a></li>
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<li><a href="https://docs.rs/tokio/latest/tokio/runtime/struct.Builder.html">Tokio Runtime Configuration</a></li>
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<li><code>/crates/vapora-backend/src/main.rs</code> (runtime entry point)</li>
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<li><code>/crates/vapora-agents/src/bin/server.rs</code> (agent runtime)</li>
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</ul>
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<hr />
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<p><strong>Related ADRs</strong>: ADR-001 (Workspace), ADR-005 (NATS JetStream)</p>
|
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</main>
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